1. Field of Invention
The present invention relates to apparatuses for measuring two-phase flow (liquid/gas) and three phase flow (liquid/liquid/gas) and related methods for reducing errors in the flow rate measurements.
2. Description of the Related Art
The application of the invention is to multiphase flow measurements in general. The teachings of the present invention can be particularly attractive for multiphase flow systems, where gas, oil and water are flowing simultaneously in the same pipe, at different velocities and concentrations.
In the last few years, many methods and apparatuses for multiphase flow metering were developed. These methods can be divided into two main groups:                Group A: Multiphase meters which are not relying on any separation devices.        Group B: Multiphase Meters where the flow is separated between gas and liquid streams, and a single-phase meter is used to measures each individual stream.Continuous developments evolved, where the separation devices' size (hence performance) was reduced and multiphase meters are used in the mostly gas and the mostly liquid lines as described in U.S. Pat. No. 5,741,977 “High void fraction multi-phase fluid flow meter” Agar Joram, Farchi David, which is hereby incorporated by reference for all purposes. This eliminated the need for perfect separation.        
The measurement of the multiphase flow in group A, utilizes an array of sensors in a single line. They rely on their difference in response to the flow, and therefore provide enough independent measurements for solving the unknown phases (oil, water and gas). A combination of momentum meters (ref. 1) is an attractive method for measuring two phase flow of liquid and gas because momentum meters (e.g. Venturi meters) measure the integral of fluid momentums (liquid and gas), and therefore are less sensitive to the homogeneity of the fluid stream. Momentum meters are also very accurate since the measurement is based on the differential pressure. Differential pressure sensors are known for their high accuracy and integrity. However, the requirement to use dissimilar momentum meters in order to get independent equations, is hard to fulfill. In reality, most momentum meters are quite similar in their behavior, since they are all based on Bernoulli's equation:
                              Δ          ⁢                                          ⁢          P                =                  k          ⁢                                          ⁢                      ρ            tp                    ⁢                                    Q              tp              2                                      2              ⁢                              A                2                                              ⁢                                    (                              1                -                                  β                  4                                            )                        .                                              Equation        ⁢                                  ⁢        1            
Thus, what are needed are methods and systems for measuring multiphase flow rates that eliminate the need of making flow rate meters dissimilar. What are also needed are flow rate meters that do not use nuclear devices or moving parts.